An automated storage and retrieval system includes one or more banks of storage cells, used for retaining stored items, one or more mechanical accessors, used to transport an item to and from a storage cell, and a controller. In an information library, for example, the stored items are recording media (such as magnetic tape cartridges or cassettes or optical cartridges, magazines or open trays holding one or more optical disks, collectively referred to herein as "cartridges"). The accessor transports cartridges between the storage cells and one or more data drives which read information from or write information to the media.
To mount a cartridge in a drive, a gripper on the accessor inserts the cartridge into a loader throat and releases its grip. The drive loader then "pulls" the cartridge into the drive and prepares to access the media. Conversely, when the drive has completed accessing the media, the loader "pushes" the cartridge partially out of the loader throat allowing the gripper to grip the cartridge, remove it from the loader throat and transport it to a storage cell. During a load operation, it is important that the drive loader be activated only after the gripper has released the cartridge. Conversely, during an unload operation, it is important that the gripper grasp the cartridge only after the drive loader has completed its action. If the gripper and the loader attempt to retain control over the cartridge simultaneously, a tug-of-war between the two occurs creating an error situation and, on occasion, causing damage to the drive or the gripper.
One proposed solution is for the drive loader and the accessor gripper to communicate with each other through a library controller. The gripper transmits a signal over a bus indicating that the cartridge is in the loader throat and is available to be pulled in. Upon receipt of the signal from the gripper, the loader, in turn, transmits a reply signal back to the gripper indicating that the loader is ready to pull the cartridge in and requesting that the gripper release the cartridge. In response, the gripper transmits a signal to the loader confirming that it has released the cartridge, whereupon the loader pulls the cartridge in for mounting. A similar set of signals is exchanged when the cartridge is to be unloaded. Such a solution, however, requires a substantial amount of bus activity, thereby degrading accessor and drive performance. Performance would be degraded further in a library having many drives and several accessors in which the exchange of signals along a library bus can be difficult to prioritize and coordinate.
Another proposed solution is for the gripper to release its grip as the cartridge is placed into the loader slot and push the cartridge in until it engages the loader (similar to the manner in which a video cassette is loaded into a VCR). However, the process of unloading a cartridge would still require an exchange of signals between the loader and the gripper or the risk of a-tug-of-war remains.